The present invention relates to a feedback network for connecting an output of an operational amplifier with a feedback input of said operational amplifier, wherein said feedback network comprises a feedback circuit.
The present invention further relates to an amplifier and/or converter circuit comprising at least one operational amplifier that comprises a feedback network for connecting an output of said operational amplifier with a feedback input of said operational amplifier, wherein said feedback network comprises a feedback circuit.
Within prior art it is well known to use operational amplifiers in a so-called closed-loop mode with a feedback network of the above mentioned type. For instance, such operational amplifiers are used as current-to-voltage converters within power meters, input stages of optical receivers and the like.
To enhance a measurement range of an input stage of an operational amplifier, it is known to provide several feedback networks which can selectively be activated and which are usually connected in parallel with each other so as to alter a resulting feedback network impedance. Activation is performed by mechanical switches such as relays, which has several disadvantages such as size, limited lifetime of relay contacts and switching speed.
It is an object of the present invention to provide an improved feedback network and an amplifier and/or converter circuit with a feedback network without the above mentioned disadvantages of prior art systems.
According to the present invention, this object is achieved by providing a semiconductor switch in series to said feedback circuit of said feedback network. Such a semiconductor switch has a very short switching time and semiconductor lifetime usually exceeds the lifetime of a mechanical relay. Apart from that, a semiconductor switch is very small and may directly be integrated on an operation amplifier""s substrate which reduces costs.
According to an advantageous embodiment of the present invention, the semiconductor switch is a field-effect transistor, which can be controlled by a control voltage and does not require a substantial control current such as mechanical relays.
A further variant of the present invention suggests that a source electrode of said field-effect transistor is connected to said feedback input of said operational amplifier, and that a drain electrode of said field-effect transistor is connected to said feedback circuit. The feedback circuit or the whole feedback network, respectively, can thus advantageously be turned on and/or off by controlling a gate electrode of said field-effect transistor.
A bulk electrode of the field-effect transistor is advantageously coupled such that a leakage current at the bulk electrode is not fed ir .o said feedback network.
To prevent leakage currents through the source electrode of the field-effect transistor, for example, a bulk electrode of said field-effect transistor is assigned the same potential as said source electrode. However, in some configurations of the operational amplifier such as basic inverting configurations, said feedback input constitutes a so-called virtual ground terminal. Consequently, said bulk electrode may not be connected to said source electrode and hence to said feedback input directly, but it must rather be connected to a real ground terminal.
According to a further very advantageous embodiment of the present invention, a correction voltage terminal is provided between said semiconductor switch and said feedback circuit. This allows for assessing a voltage drop across the semiconductor switch that affects a feedback voltage fed back to said feedback input of said operational amplifier.
From said correction voltage terminal, within a further embodiment of the invention, a correction voltage is obtained with which an output voltage of said operational amplifier can be corrected. To eliminate the non-linear influence of said voltage drop across the semiconductor switch on said output voltage, said correction voltage is subtracted from said output voltage.
A further variant of the invention provides a photodiode connected to said feedback input of said operational amplifier which enables to process optical signals.
As a further solution to the object of the present invention, an amplifier and/or converter circuit is presented which comprises at least one operational amplifier that comprises a feedback network for connecting an output of said operational amplifier with a feedback input of said operational amplifier, wherein said feedback network comprises a feedback circuit and a semiconductor switch in series to said feedback circuit.
Within a very advantageous embodiment of said amplifier and/or converter, said operational amplifier(s) comprise(s) a plurality of said feedback networks according to the present invention which are connected in parallel to each other. With this configuration, it is possible to simultaneously turn on and/or turn off, i.e. select an arbitrary number of feedback networks thus altering a resulting feedback network impedance between said output of said operational amplifier and said feedback input. Changing the feedback network impedance influences a gain factor of said amplifier/converter and thus contributes to an enhancement of a measurement range of said amplifier.
Yet another advantageous variant of the invention is characterized by a subtractor circuit for subtracting said correction voltage from said output voltage of said operational amplifier. Said subtractor circuit may be implemented as an operational amplifier in subtractor configuration.
The feedback network and the amplifier and/or converter given by the present invention is particularly well-suited for an input stage of an optical power meter and for controlling and enhancing, respectively, a measurement range of said input stage. Generally, the amplifier/converter according to the present invention is ideally suited for applications which require a processing of input signals with a high dynamic range.